When Friend leukemia cells are deprived of a single essential amino acid, the relative level of the specific tRNAs cognate to the deprived amine acid increases significantly. This effect is mediated by a specific stabilization of cognate tRNAs towards intracellular degradation. These observations suggest that the functional adaptation of the tRNA population which is known to occur in numerous differentiating systems is induced by changes in the extent of aminoacylation of tRNA species produced by changes in the pattern of amino acid utilization. In order to test this theory, we plan to determine whether the same mechanism is used to regulate specific tRNA levels in differentiating Friend leukemia cells as is used in cells deprived of a single essential amino acid. Additionally, we shall investigate whether coordinate regulation of aminoacyl-tRNA synthetases and their cognate tRNAs occurs in these situations.